A number of catalyst systems are known for the reactions between a vinyl aromatic compound and an .alpha.-monoolefin. Examples are the use of rhodium chloride in the reaction of styrene with ethylene (U.S. Pat. No. 3,013,066) and the use of palladium chloride in the same reaction (J. Orgmet. Chem. 21, 1970. 218). However, the yields of the products obtained by these processes have not been sufficiently high per industrial purposes.
U.S. Pat. No. 3,803,254 discloses a process of cooligomerization of a vinyl aromatic monomer with an .alpha.-monoolefin, in the presence of a cationic palladium complex prepared by treating a palladium compound with a reagent containing a complex fluoro anion or a perchlorate anion and with a trivalent organo phosphorus ligand. This palladium complex is inherently unstable, and in order to counter catalyst decomposition by precipitation of metallic palladium the patent recommends the addition of a Lewis acid.
The preparation and maintenance of these catalyst systems are relatively cumbersome and expensive.
In EP-A-0376364, a process is disclosed for the preparation of polymers of carbon monoxide with one or more .alpha.-olefins having at least three carbon atoms in the molecule, whereby a catalyst composition is used which is based upon:
a) a palladium compound, PA0 b) an anion of an acid with a pKa of less than 2, and PA0 c) a diphosphine of the general formula R.sup.1 R.sup.2 P-X-PR.sup.3 R.sup.4, in which , R.sup.1, R.sup.2, R.sup.3 and R.sup.4 represent the same or different optionally polar-substituted aliphatic hydrocarbyl groups and X is a bivalent organic bridging group containing at least two carbon atoms in the bridge. PA0 a) a palladium compound, PA0 b) an anion of an acid with a pKa of less than 2, and PA0 c) a diphosphine of the general formula in which R.sup.1 R.sup.2 P-X-PR.sup.3 R.sup.4, in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4 represent the same or different unsubstituted or polar-substituted aliphatic hydrocarbyl groups and X is a bivalent organic bridging group containing at least two carbon atoms in the bridge.
It has now been found that this catalyst composition can be used to great advantage in the process for the co-dimerization of a vinyl aromatic monomer with an o-monoolefin monomer or a derivative thereof, giving good conversion and excellent selectivity, and that the catalyst composition is very simple to prepare and very stable in this co-dimerization reaction.